This study is concerned with the relationship of calcitonin to phosphate homeostasis, and the correlation of this to the hypocalcemic action of the hormone. The study has also been concerned with the relationship of the gastrointestinal tract to calcitonin secretion and the role of calcitonin in the conservation of dietary calcium. It is concluded that calcitonin secretion during feeding permits the continued secretion of parathyroid hormone, and the two hormones in concert reduce urinary loss of calcium. The other action of calcitonin under investigation is its role in conservation of body stores of phosphate. Results of the past year suggest that calcitonin moves phosphate into bone and prevents its loss from bone when hypophosphatemic conditions exist. This aspect will be the main emphasis of next year's study which will include ultrastructural studies of the area in bone believed to be most sensitive to calcitonin action.